Semimetallic resistive filaments



July29, 5 M. L. ANDERSON ETAL 2,845,515

SEMIMETALLIC RESIS'iIVE FILAMENTS Filed March 12, 1953 CONDUCTIVE PARTICLES INSULATING CONDUCTOR F76, PARTICLES /5\|NSULAT|NG COATING CONDUCTOR 5N no w L N TR N R W TR M United States Patent 2,845,515 SEMIMETALLIC RESISTIVE FILAMENTS "Mauritz :L. Anderson, Arlington, and Frederick T. mu,

Bedford, Mass., assignors to Raytheon l l-ianufacturing Company, Newton, Mass., a corporation of Delaware This invention relates to resistive filaments of a type that can be used for heating the cathodes of thermionic tubes and more particularly to such resistive filaments made of a mixture of conductive and dielectric refractory materials.

Heater filaments for indirectly-heated cathodes of thermionic tubes have been made by coating a conductor with an insulating material and inserting the insulated conductor into the cathode sleeve. With such filaments it has been found that the insulating coating is likely to break away at the bend under vibration and shock. When this occurs, the filament is likely to come into contact with the cathode sleeve, producing undesirable results in the operation of the tube.

These possibilities are avoided by the construction of this invention where the heater is composed of finelydivided conductive and refractory dielectric particles with conductors embedded one in each end to serve as connections to the external circuit and the body of the resistor covered with an insulating coating. A heater resistor formed in this manner can be mounted in a cathode sleeve with less danger of heater-to-cathode shorts occurring when the tube is subjected to vibration and shock.

These and other features and advantages of the invention will become more apparent in the following descrip tion taken in connection with the accompanying drawings illustrating the invention wherein:

Fig. 1 is an enlarged longitudinal view of a resistive heater filament of the invention shown partly broken away; and

Fig. 2 is a view in elevation of a thermionic tube made with a resistive heater filament of the invention shown partly broken away.

In Fig. 1, the reference numeral refers to the body of the resistive filament composed of a mixture of particles 11 of a conductive material and particles 12 of an insulating refractory material. The conductive material is preferably a metal having a high melting point and nonreactive with the refractory material used at the operating temperature of the tube, that is, at temperatures in the order of 1100 to 1200 degrees C. Suitable metals for this purpose are tungsten, titanium, tantalum and molybdenum. Mixtures of these metals may also be used. The refractory material is preferably alumina, magnesium oxide, zirconium oxide or some other oxide or silicon carbide or nitride or a mixture of one or more of these materials. The average particle size of both the conductive and refractory materials should be as small as possible, preferably in the order of four to five microns in diameter.

The materials are mixed in the proportion of at least about forty percent of conductive particles. In the case of molybdenum, the metallic content should be fortythree percent of the mixture. In the case of tungsten and tantalum, the metallic content should be about fiftythree percent of the mixture. It has been found that, if the mixture comprises fifty percent or less of tungsten, the resulting filament will have substantially infinite resistance, while, if the mixture contains more than fifty-five percent of tungsten, the resulting resistor will have a conductivity approximately equal to that of pure tungsten. Tantalum givessimilar results. When the refractory material contains conductive material, such as nitrides and carbides of titanium, silicon, tungsten or boron, the

conductivity of the resulting resistor will be increased unless the proportion of metallic particles in the mixture is reduced. Water and other extrusion aids, such as flour paste, waxes, bentonite clay, gums or resins, are added to the mixture to the extent of three or four percent of the total to form a thick paste which is then formed by extrusion into the desired shape, usually a long, slender cylinder. Representative resistors of this type have been formed with a diameter of .03 inch.

Thin leads 13 and 14 of tungsten or other conductive material of a convenient length and diameter, such as .003 inch, are inserted one into each end of the formed resistor to a depth of penetration sufiicient to assure good conduct with the body of the resistor. The formed resistor with the leads inserted is dried and baked at a temperature of about 1650 degrees C., preferably in a hydrogen or other reducing atmosphere. In the course of the baking, the extrusion aids, with the exception of any clay present, are driven out and other changes take place, causing the body to shrink about the embedded leads, gripping them securely.

The baked resistor is then covered with a coating 15 of alumina or other insulating material applied as a paste to form an insulating coating. The resistor is again baked to harden this coating.

Such a resistor may be mounted in a cathode sleeve, as shown in Fig. 2. The resistor is mounted within a cathode sleeve 16 between mica spacers 17 and 18 within a tube envelope 20. One lead 13 is brought through the seal of the envelope to a prong 21. The other lead 14 is welded to a conductive supporting rod 22 that is brought through the seal of the envelope. A grid 23 and an anode 24 are mounted concentrically about the oathode 16 within the envelope 20. A resistor of the type described mounted in a cathode sleeve in the above-described manner or in any other convenient way forms an indirect heater for a cathode that is highly resistant to shock and vibration and contributes greatly to the reliability of the tube in which it is used.

This invention is not limited to the particular details of construction, material and processes described, as many equivalents will suggest themselves to those skilled in the art.

What is claimed is:

l. A resistive filament for a cathode of a thermionic discharge device consisting of a mixture of about forty to fifty-five percent of finely-divided molybdenum and the balance of dielectric refractory particles formed into a self-supporting body and covered by an insulating coating, and conductors inserted into the body of the filament at each end of said filament.

2. A resistive filament for a cathode of a thermionic discharge device consisting of a mixture of forty-three percent of finely-divided molybdenum and the balance of dielectric refractory particles formed into a self-supporting body and covered by an insulating coating, and conductors inserted into the body of the filament at each end of said filament.

3. A resistive filament for a cathode of thermionic discharge device consisting of a mixture of forty to forty-five percent finely-divided molybdenum and alumina particles formed into a self-supporting body and covered by an insulating coating, and conductors inserted into the body of the filament at each end of said filament.

4. A resistive filament for a cathode of a thermionic discharge device consisting of a mixture of forty to fortyfive percent finely-divided molybdenum and silicon carbide particles forrned into a self-supporting body and covered by an insulating coating, and conductors inserted into the body of the filament at each end of said filament.

References Cited in the file of this patent UNITED STATES PATENTS 4 Browne et a1. Dec. 4, 1934 Randall Feb. 5, 1935 Sklar Mar. 19, 1935 Andre Mar. 9, 1937 Pearson Oct. 14, 1941 Megow et a1 Dec. 22, 1942 Haberberger Oct. 19, 1943 Buritz et a1. Ian. 21, 1947 Toorks Dec. 20, 1947 Miller Ian. 11, 1949 Loosjes et a1 Oct. 21, 1952 

1. A RESISTIVE FILAMENT FOR A CATHODE OF A THERMIONIC DISCHARGE DEVICE CONSISTING OF A MIXTURE OF ABOUT FORTY TO FIFTY-FIVE PERCENT OF FINELY-DIVIDED MOLYBDENUM AND THE BALANCE OF DIELECTRIC REFRACTORY PARTICLES FORMED INTO A SELF-SUPPORTING BODY AND COVERED BY AN INSULATING COATING, AND CONDUCTORS INSERTED INTO THE BODY OF THE FILAMENT AT EACH END OF SAID FILAMENT. 